Alcohols have a variety of applications in industry and science such as a beverage (i.e., ethanol), fuel, reagents, solvents, and antiseptics. For example, butanol is an alcohol that is an important industrial chemical and drop-in fuel component with a variety of applications including use as a renewable fuel additive, as a feedstock chemical in the plastics industry, and as a food-grade extractant in the food and flavor industry. Accordingly, there is a high demand for alcohols such as butanol, as well as for efficient and environmentally-friendly production methods.
Production of alcohol utilizing fermentation by microorganisms is one such environmentally-friendly production method. In the production of butanol, in particular, some microorganisms that produce butanol in high yields also have low butanol toxicity thresholds. Removal of butanol from the fermentation vessel as it is being produced is a means to manage these low butanol toxicity thresholds. Thus, there is a continuing need to develop efficient methods and systems for producing butanol in high yields despite low butanol toxicity thresholds of the butanol-producing microorganisms in the fermentation medium.
In situ product removal (ISPR) (also referred to as extractive fermentation) can be used to remove butanol (or other fermentative alcohol) from the fermentation vessel as it is produced, thereby allowing the microorganism to produce butanol at high yields. One ISPR method for removing fermentative alcohol that has been described in the art is liquid-liquid extraction (U.S. Patent Application Publication No. 2009/0305370). In general, with regard to butanol fermentation, the fermentation medium which includes the microorganism is contacted with an organic extractant at a time before the butanol concentration reaches a toxic level. The organic extractant and the fermentation medium form a biphasic mixture. The butanol partitions into the organic extractant phase decreasing the concentration of butanol in the aqueous phase containing the microorganism, thereby limiting the exposure of the microorganism to the inhibitory butanol. In order to be technically and economically viable, liquid-liquid extraction requires contact between the extractant and the fermentation broth for efficient mass transfer of the product alcohol into the extractant; phase separation of the extractant from the fermentation broth (during an/or after fermentation); efficient recovery and recycle of the extractant; and minimal decrease of the partition coefficient of the extractant over a long-term operation.
The extractant can become contaminated over time with each recycle, for example, by the build-up of lipids present in the biomass that is fed to the fermentation vessel as feedstock of hydrolyzable starch. As an example, during the conversion of glucose to butanol, a liquified corn mash loaded to a fermentation vessel at 30 wt % dry corn solids can result in a fermentation broth that contains about 1.2 wt % corn oil generated by simultaneous saccharification and fermentation (with saccharification of the liquified mash occurring during fermentation by the addition of glucoamylase to produce glucose). The dissolution of the corn oil lipids into oleyl alcohol (OA) serving as an extractant during ISPR can result in build-up of lipid concentration with each OA recycle decreasing the partition coefficient for the product alcohol in OA as the lipid concentration in OA increases with each recycle of OA.
In addition, the presence of the undissolved solids during extractive fermentation can negatively affect the efficiency of the alcohol production. For example, the presence of the undissolved solids may lower the mass transfer coefficient inside the fermentation vessel, impede phase separation in the fermentation vessel, result in the accumulation of corn oil from the undissolved solids in the extractant leading to reduced extraction efficiency over time, increase the loss of solvent because it becomes trapped in solids and ultimately removed as Dried Distillers' Grains with Solubles (DDGS), slow the disengagement of extractant drops from the fermentation broth, and/or result in a lower fermentation vessel volume efficiency.
Several approaches for reducing the degradation of the partition coefficient of the extractant used in extractive fermentation have included wet milling, fractionation, and removal of solids. Wet milling is an expensive, multi-step process that separates a biomass (e.g., corn) into its key components (germ, pericarp fiber, starch, and gluten) in order to capture value from each co-product separately. This process gives a purified starch stream; however, it is costly and includes the separation of the biomass into its non-starch components which is unnecessary for fermentative alcohol production. Fractionation removes fiber and germ, which contains a majority of the lipids present in ground whole corn resulting in a fractionated corn that has a higher starch (endosperm) content. Dry fractionation does not separate the germ from fiber and therefore, it is less expensive than wet milling. However, fractionation does not remove the entirety of the fiber or germ, and does not result in total elimination of solids. Furthermore, there is some loss of starch in fractionation. Wet milling of corn is more expensive than dry fractionation, but dry fractionation is more expensive than dry grinding of unfractionated corn. Removal of solids including germ containing lipids, from liquefied mash prior to use in fermentation can substantially eliminate undissolved solids as described, for example, in co-pending, commonly owned U.S. Provisional Application Ser. No. 61/356,290, filed Jun. 18, 2010. However, it would be advantageous if the degradation of the partition coefficient of the extractant caused by contamination by lipid can be reduced even without fractionation or removal of substantially all undissolved solids. Converting the lipids present in a liquefied mash into an extractant that can be used in ISPR is another method of decreasing the amount of lipids that are fed to the fermentation vessel as described, for example, in co-pending, commonly owned U.S. Provisional Application Ser. No. 61/368,436 and U.S. Provisional Application Ser. No. 61/368,444, both filed on Jul. 28, 2010.
There is a continuing need for alternative extractive fermentation methods which do not necessitate the partitioning of the product alcohol between the fermentation medium and the ISPR extractant as a means to reduce the toxic effect of the product alcohol such as butanol on the microorganism, and which can also reduce the degradation of the partition coefficient of a fermentation product extractant.